Graphite-silicon carbide composite and making method

ABSTRACT

A graphite-silicon carbide composite comprises a graphite substrate and a silicon carbide layer formed thereon and comprising silicon carbide particles in fused and contact bonded state. The composite has excellent oxidation resistance and finds a wide range of application as heat resistant material. The method of forming a silicon carbide layer on graphite surface is simple and consistent.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of co-pending application Ser. No.12/055,985, filed on Mar. 26, 2008, the entire contents of which arehereby incorporated by reference and for which priority is claimed under35 U.S.C. §120. Priority under 35 U.S.C. §119(a) is also claimed onPatent Application No. 2007-084385 filed in Japan on Mar. 28, 2007, theentire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to graphite-silicon carbide composites which areapplicable even in an oxidizing atmosphere as high-temperaturestructures, fixtures, semiconductor equipment members, liquid crystalequipment members, mechanical sliders and the like, and a method forpreparing the same.

BACKGROUND ART

Because of excellent high-temperature properties, mechanical strengthand workability, graphite materials find use as a variety ofhigh-temperature materials. However, graphite materials are lessresistant to oxidation and thus limited to use in a non-oxidizingatmosphere. The high-temperature materials which can be used in anoxidizing atmosphere are oxide ceramics including silicon carbide,silicon nitride and alumina. However, these ceramics suffer from severalproblems including inefficient working, difficult size enlargement, andpoor thermal shock resistance.

Then, for improved oxidation resistance, an attempt was made to producea graphite-silicon carbide composite by coating the surface of graphitewith a silicon carbide layer. Several methods are known for thepreparation of graphite-silicon carbide composites. For instance, JP-B61-11911 discloses a method for preparing a silicon carbide-graphitecomposite by providing a carbon substrate in which micro-pores having aspecific diameter occupy a volume of at least 0.02 cm³/g and effectingconversion using SiO gas. JP-A 62-132787 discloses a method forpreparing a silicon carbide-graphite composite by providing a poroussilicon carbide sintered body having an open porosity of 5 to 55% and anaverage pore size of 1 to 100 μm and filling open pores with carbon. JPPatent 2620294 discloses a method for preparing a siliconcarbide-graphite composite by infiltrating molten silicon into a porousgraphite substrate and effecting reaction.

However, these prior art methods involve complicated steps and result inlow yields of manufacture, which means that the resultinggraphite-silicon carbide composites are expensive. Substantialvariations of silicon carbide coating and inconsistent quality ofproducts are also problems. These production methods are thus notregarded as industrially efficient.

DISCLOSURE OF THE INVENTION

An object of the invention is to provide a graphite-silicon carbidecomposite which is resistant to a high-temperature oxidizing atmosphere,useful as heat resistant material, and minimized in the variation ofquality, and a method for preparing the same.

The inventors have found that by thermally spraying metallic siliconhaving a selected particle size onto a surface of a graphite substrateand heat treating the coated substrate, a silicon carbide layer having adesired thickness with minimized variation can be readily formed on thegraphite substrate surface, and that the resulting graphite-siliconcarbide composite tolerates use in a high-temperature oxidizingatmosphere.

In one aspect, the invention provides a graphite-silicon carbidecomposite comprising a graphite substrate having a surface and a siliconcarbide layer formed on the surface. The silicon carbide layer iscomposed of silicon carbide particles in fused and contact bonded state.

In preferred embodiments, the silicon carbide particles have an averageparticle size of 0.5 μm to 50 μm, and the silicon carbide layer has athickness of 10 μm to 300 μm. Typically the graphite-silicon carbidecomposite has a gas permeability equal to or less than 1.0×10⁻² cm²/s.

In another aspect, the invention provides a method for preparing agraphite-silicon carbide composite comprising the steps of thermallyspraying a metallic silicon powder onto a surface of a graphitesubstrate, and heat treating the sprayed substrate in a non-oxidizingatmosphere at a temperature of 1100° C. to 1700° C. for forming on thesubstrate surface a silicon carbide layer comprising silicon carbideparticles in fused and contact bonded state. Preferably, the metallicsilicon powder has an average particle size of 0.5 μm to 50 μm.

BENEFITS OF THE INVENTION

The graphite-silicon carbide composite of the invention has excellentoxidation resistance and finds a wider range of various applications asheat resistant material. The method of forming a silicon carbide layeron graphite surface is simple and consistent enough to ensure productionof graphite-silicon carbide composites with minimized variation inquality and to enable efficient manufacture on an industrial scale.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The graphite-silicon carbide composite of the invention comprises agraphite substrate and a silicon carbide layer formed thereon. Thesilicon carbide layer consists of silicon carbide particles in fused andcontact bonded state. The “silicon carbide particles in fused andcontact bonded state” means that silicon carbide in the fused stateforms a junction or bond between graphite substrate and silicon carbideor between silicon carbides together by an impetus toward the graphitesubstrate in a perpendicular direction thereto. Specifically, siliconcarbide takes an ellipsoidal shape and forms a junction or bond insurface contact rather than in point contact.

The graphite-silicon carbide composite is prepared by thermally sprayinga metallic silicon powder onto a surface of a graphite substrate andheat treating the sprayed substrate in a non-oxidizing atmosphere at atemperature of 1100° C. to 1700° C.

The graphite substrate used herein is not particularly limited. A choicemay be made among cold isostatic press (CIP) molded parts, extrusionmolded parts and carbon/carbon (C/C) composites, depending on anintended application. Of these, C/C composites are advantageously usedfor high strength. The shape and size of graphite substrate are notparticularly limited. It is understood that C/C composites are compositematerials formed from carbon fibers and graphite particles and havinghigh strength and brittleness. The mixing ratio of carbon fibers tographite particles is generally in a range between 7/3 and 3/7.

Next, silicon powder is thermally sprayed on the graphite substrate. Thespraying method is not particularly limited and includes plasmaspraying, combustion flame spraying using acetylene, propane or keroseneas the fuel gas, and high-velocity flame spraying. Silicon powder is fedinto a plasma flame or gas flame whereby silicon in semi-fused state issprayed to the graphite substrate. Of these, the plasma spraying isadvantageously used because a coating of better adhesion can be formedat higher temperature.

The silicon powder to be sprayed is not particularly limited. A choicemay be made among silicon powders of the semiconductor, ceramic andchemical grades, depending on an intended application. Although theparticle size of silicon powder is not particularly limited as well, anaverage particle size of 0.5 to 50 μm is desired, and more desirably 3to 30 μm. A powder with an average particle size of less than 0.5 μm maybe difficult to spray, with a uniform spray being not expectable. Apowder with an average particle size of more than 50 μm can be sprayed,but may hinder its conversion into silicon carbide by heat treatment,resulting in a silicon carbide layer on the graphite substrate surfacecontaining more unreacted silicon powder.

It is noted that the “average particle size” refers to a weight averagevalue D₅₀ when the particle size distribution is determined by a laserdiffraction technique, i.e., a particle size when the cumulative weightreaches 50% (also referred to as median particle size).

Particles of silicon carbide formed by spraying the silicon powder andsubsequent heat treatment also have an average particle size of 0.5 to50 μm, and more desirably 3 to 30 μm. The average particle size ofsilicon carbide particles is evaluated as a value corresponding to theaverage particle size of silicon powder.

The size of SiC particles may be measured by sedimentation, imageanalysis, laser diffraction or other techniques. Herein, a particle sizeas determined by the laser diffraction technique is used for quickmeasurement and high reproducibility.

Once the silicon powder is sprayed on the graphite substrate surface,the sprayed substrate is heat treated to form a silicon carbide layer onits surface. Heat treatment is at a temperature of 1100° C. to 1700° C.and preferably 1200° C. to 1500° C. A heat treatment temperature below1100° C. achieves a low percent conversion of silicon powder to siliconcarbide, resulting in a silicon carbide layer containing more unreactedsilicon powder. If the heat treatment temperature exceeds 1700° C.,which is far beyond the melting point of silicon powder, the sprayedsilicon powder is thoroughly melted, resulting in a graphite-siliconcarbide composite having a silicon carbide layer with noticeably varyingthickness.

As long as the atmosphere where heat treatment is carried out is anon-oxidizing atmosphere, no other considerations are necessary. Heattreatment may be carried out in an inert gas such as Ar or He and underatmospheric or reduced pressure. The apparatus for carrying out heattreatment is not particularly limited as well, and a batch furnace,continuous tunnel furnace or the like may be used.

The sprayed coating of silicon powder preferably has a thickness of 10to 300 μm, and more preferably 10 to 200 μm, although the thickness isnot particularly limited. Correspondingly, the silicon carbide layer ofthe graphite-silicon carbide composite also preferably has a thicknessof 10 to 300 μm, and more preferably 10 to 200 μm. If the thickness isless than 10 μm, the silicon carbide layer may have a lower gaspermeability and not tolerate long-term service in a high-temperatureoxidizing atmosphere. Inversely, if the thickness is more than 300 μm,no improvement in gas permeability is observed and an increased spraycost is the only result. Since the thickness of silicon carbide layercan be controlled by the thickness of a coating of silicon powdersprayed, a predetermined thickness is readily achievable.

Preferably, the graphite-silicon carbide composite has a gaspermeability equal to or less than 1.0×10⁻² cm²/s and more preferablyequal to or less than 1.0×10⁻³ cm²/s. If the gas permeability is morethan 1.0×10⁻² cm²/s, oxygen in the ambient atmosphere can reach thegraphite matrix to detract from the oxidation resistance of thecomposite. Since the gas permeability can be controlled by the thicknessof silicon carbide layer, a gas permeability of 1.0×10⁻² cm²/s or lessis achievable by setting the thickness of silicon carbide layer to 10 μmor greater.

It is noted that the gas permeability is determined according to Darcyequation by measuring a volume of gas flow through a specimen when apressure difference ΔP is established across the specimen.

K=QL/ΔPA

K: gas permeability (cm²/s)

Q: volume of gas flow (Pa-cm³/s)

ΔP: pressure difference across specimen (Pa)

L: specimen thickness (cm)

A: gas permeation area (cm²)

EXAMPLE

Examples of the invention are given below by way of illustration and notby way of limitation.

Example 1

On entire surfaces of a C/C composite plate of 100 mm×100 mm×5 mm(thick), a metallic silicon powder having an average particle size of 20μm was plasma sprayed to form a silicon powder coating of 50 μm thick.The coated plate was then placed in a batch furnace where it was heattreated in a reduced pressure and at 1450° C. for 5 hours.

For the product, a cross section of the substrate was observed and thesurface layer was analyzed by X-ray diffraction microscopy. It was agreen graphite-silicon carbide composite in which particulate siliconcarbide having an average particle size of 20 μm had been fused andcontact bonded. The composite had a gas permeability of 1.0×10⁻⁵ cm²/s

The graphite-silicon carbide composite was evaluated for oxidationresistance. It was held in air at 800° C. for 3 hours, and then cooleddown. The weight was measured to find a weight loss of −0.1 wt %. Aweight change of substantially zero proved it to be a fully oxidationresistant material.

Comparative Example 1

A C/C composite plate without a silicon carbide layer was subjected tothe oxidation resistance test as in Example 1. It is noted that the C/Ccomposite plate had a gas permeability of 5.0×10 ⁻¹ cm²/s. A weight losson heating of −88 wt % was found, indicating inferior heat resistance toExample 1.

Japanese Patent Application No. 2007-084385 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A method for preparing a graphite-silicon carbide compositecomprising the steps of: thermally spraying a metallic silicon powderonto a surface of a graphite substrate, and heat treating the sprayedsubstrate in a non-oxidizing atmosphere at a temperature of 1100° C. to1700° C. for forming on the substrate surface a silicon carbide layercomprising silicon carbide particles in fused and contact bonded state.2. The method of claim 1, wherein the metallic silicon powder has anaverage particle size of 0.5 μm to 50 μm.
 3. The method of claim 1,wherein the temperature of heat treating is 1200° C. to 1500° C.
 4. Amethod of preparing a graphite-silicon carbide composite comprising thesteps of: plasma spraying a metallic silicon powder having an averageparticle size of 3 μm to 30 μm onto a surface of a carbon/carboncomposite substrate composed of carbon fibers and graphite particleshaving a mixing ratio of between 7/3 and 3/7 in a thickness of 10 to 300μm, and heat treating the sprayed substrate in a non-oxidizingatmosphere at a temperature of 1100° C. to 1700° C. for forming on thecarbon/carbon composite substrate a silicon carbide layer comprisingsilicon carbide particles in fused and contact bonded state, therebyobtaining the graphite-silicon carbide composite having a gaspermeability equal to or less than 1.0×10⁻² cm²/s.
 5. The method ofclaim 4, wherein the temperature of heat treating is 1200° C. to 1500°C.
 6. The method of claim 4, wherein the metallic silicon powder issprayed in a thickness of 10 to 200 μm.
 7. The method of claim 4,wherein the gas permeability of the graphite-silicon carbide compositeis equal to or less than 1.0×10⁻³ cm²/s.